The advantages of semiconductor lasers are the direct and highly efficient conversion of electric power into coherent light, compactness, and the capability of direct power and frequency modulation. However, in contrast to other types of lasers (for example, gas lasers or YAG: Nd lasers), semiconductor lasers do not provide a collimated beam without complex optical focusing systems.
Typically, a semiconductor laser is formed as a double heterostructure. (see e.g., Botez "Laser Diodes are Power Packed" IEEE Spectrum June 1985 pp. 43-53) The double-heterostructure semiconductor laser comprises a semiconductor body having first and second relatively wide hand-gap cladding layers of opposite conductivity type, and a relatively narrow bandgap active layer which is located between the cladding layers. The layers are grown on a suitable substrate. Illustratively, the narrow bandgap active layer comprises GaAs and the wide bandgap cladding layers comprise Al.sub.x Ga.sub.1-x As where x is about 0.35. The substrate is GaAs. Generally, electrical contacts are provided on the top and bottom surfaces of the semiconductor body comprising the laser so that the resulting diode structure can be forward biased.
When the structure is forward biased, electrons and holes from the cladding layers are injected into the active layer where radiative recombination takes place. The radiation occurs within a wavelength band determined by the bandgap of the active layer. Normally, the active layer is highly absorbing. However when a feedback mechanism is present, lasing takes place. Typically, the feedback is provided by making the end facets of the laser at least partially reflecting. In this case, radiation emitted in the active layer travels back and forth between the partially reflecting end facets of the semiconductor body. As the forward bias pumping current is increased, absorption is diminished and is replaced by amplification. Lasing begins when the round trip optical gain exceeds losses due to mechanisms such as absorbtion, scattering, and facet transmission.
In alternative lasers, structures other than reflecting end facets may be used to supply the necessary feedback. For example, a distributed Bragg reflector located outside the semiconductor body may be utilized to supply feedback. (see e.g. Olsson et al "Performance Characteristics of a 1.5 .mu.m Single-Frequency Semiconductor Laser with an External Waveguide Bragg Reflector", IEEE Journal of Quantum Electronics Vol. 24, No. 2, February 1988 pp. 143-147; Olsson et al "Narrow Linewidth 1.5 .mu.m Semiconductor Laser With a Resonant Optical Reflector" Applied Physics Letters Vol. 51, No. 15, Oct. 12, 1987, pp. 1141-1142; Kazarinov et al "The Relation of Line Narrowing and Chirp Reduction Resulting from the Coupling of a Semiconductor Laser to a Passive Resonator" IEEE Journal of Quantum Electronics, Vol. QE-23, No. 9, September 1987 pp. 1401-1409)
In a double-heterostructure laser structure, the index of refraction of the active layer is larger than the index of refraction of the surrounding cladding layers. Thus, the emitted radiation is transversely confined in a one-dimensional dielectric waveguide formed by the two cladding layers and the active layer. For devices with active layers thinner than about 0.3 micrometers, the arrangement is such that only the fundamental transverse mode is supported. (As used herein, transverse means perpendicular to the plane of the layers comprising the laser).
While the light is guided in the lowest order mode in the transverse direction, such is not normally the case in the lateral direction (i.e. in the plane of the layers). If a relatively wide stripe contact is used to inject the pumping current, the optical output exhibits unstable multimode behavior in the lateral direction. This unstable behavior is exaggerated as one goes to higher and higher power. Thus, such wide stripe structures, although they can produce the desired high power, have heretofore proven unsuitable for use in the typical applications contemplated for high power semiconductor diode lasers, which applications require a single stable lowest order mode optical beam.
Various techniques have been developed to provide for the confinement of emitted radiation in the lateral direction so as to achieve stable and reliable laser operation in the lowest order lateral mode. The simplest technique involves use of a narrow stripe contact on the upper surface of the semiconductor body forming the laser. In this case, the profile of injected carriers forms a weak waveguide which provides some guiding in the lateral direction. This type of laser is known as a gain guided laser. A narrow stripe gain-guided laser structure exhibits unstable multibeam behavior at high power. Other techniques for achieving confinement of emitted radiation in the lateral direction involve the formation of complex dielectric waveguide structures in the lateral direction. This type of laser is known as an index guided laser. An example of an index guided laser is the Buried-Heterostructure laser. Besides their complex fabrication procedures, a further weakness of such index guided lasers is that the power output of such lasers is severely limited because a power output density in excess of 6 to 9 mw per square micrometer at an emitting facet causes the facet to be damaged and the laser performance to degrade.
Other attempts to achieve high power fundamental lateral mode laser operation involve the phase synchronization of laser arrays (see e.g., D. R. Scifers et al., "High Power Plasma Array Lasers" Proc. of Eighth IEEE International Semiconductor Laser Conference (1982), Ottawa pp.22-23) and the use of a conventional spatial filter. Both of these approaches produce highly asymmetrical elliptic beams.
In view of the foregoing, it is an object of the present invention to provide a semiconductor structure (a) which operates in the fundamental lateral mode at high power, (b) which operates at a single frequency, and which c) provides a highly collimated and symmetrical output beam.